I may have discovered a bug in the resizing behaviour for background images.
Method 1
1. Click on the background image.
2. In the panel on the left hand side choose Detach page background.
3. When you've finished your edit, choose Set as page background if you want to, or leave as a simple layer.
When you do these steps and choose Set to background the image reverts to the original format.
Method 2
1. You can also double-click the image background which will open the Image panel and give you the blue bounding box and control handles.
2. You can resize your image as desired and close the image panel and your background has resized but remains a background as opposed to changing to a layer.
Using this method the image changes remain "sticky" and do not revert.
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